Why is the general recommendation for protecting IC input from transient spikes to use a diode pair like this article shows in the 'cpu dataline protection' figure. Diode has a forward voltage drop that prevents my input to see rail to rail levels at the IC. Is there another recommendation?

http://www.electronics-tutorials.ws/diode/diode_4.html

 

diode pair is reverse biased and does NOT prevent your input from going rail to rail. it prevents it from going outside of the range (0-Vd, Vcc+Vd) where Vd=0.7V approx. therefore if Vcc is 5V, diodes would clamp the signal to range -0.7V to 5.7V.

http://www.electronics-tutorials.ws/diode/diode9a.gif

 

Oops. Yes, your right.

I meant to ask what if I can't pass levels > 5.0 and < 5.7 to the input of my IC? What should I do?

 

not sure what you are asking.... are you asking how to limit signal to be within 0..5V and not -0.7..5.7V? what would be concern to ask something like this? do you only care about upper limit to be at 5V? you can get rid of the diodes and add zener 5.1V in parallel with input. or... you can replace both diodes by 5.1V zeners.

 

Oops. Yes, your right.

I meant to ask what if I can't pass levels > 5.0 and < 5.7 to the input of my IC? What should I do?

Most IC's can tolerate 0.7V outside of their range. If your's can't, then there are diodes with lower forward bias voltages around. Or, as pointed out already, you can use a 5.1V zener diode from the signal line to ground (reverse biased) to clamp it at only about 100mV above Vcc. But be sure that the current that is starting to flow at 5V, which can be quite noticeable that close to the knee, doesn't disturb the signal too much.

Short of that, you could probably use an active "ideal diode" to protect the circuit.

 

you can use a 5.1V zener diode from the signal line to ground (reverse biased)

Should I put a resistor between zener and ground?

But be sure that the current that is starting to flow at 5V, which can be quite noticeable that close to the knee, doesn't disturb the signal too much.

Do you mean between 5~5.1V things get weird? Wouldn't anything below 5.1V just flow to IC input as normal? Thanks!

 

before we toss dozens of ideas around and design something overly elaborate and complicated, why don't you just tell us what exactly you want to do?

 

Should I put a resistor between zener and ground?

Then the zener will lose its function.

Do you mean between 5~5.1V things get weird? Wouldn't anything below 5.1V just flow to IC input as normal? Thanks!

As the TTL IC can be provided the power 5V±5%=4.75~5.25V.
If Vcc<4.75V then IC will not work properly, if Vcc>5.25V could damage the IC, so under 5.1V should be ok for the IC, and don't care about the 5~5.1V.

 

The thread is "signal clipping"..
Yet Diode clamping for Logic Levels is being discussed....
Am I confused, or what?
I agree that using diode clamps for the input to audio ICs is a "bad idea" and will cause signal clipping (which is why it is not a generally accepted practice). But on the other hand, Logic levels are square waves (which is what a diode clipper will emulate when used with an audio signal with high peaks - in which the diodes will clip - giving the audio waveform flat tops and bottoms "ie: clipping").
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I agree with panic_mode the desires of the oP needs clarification.
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I again wish everyone a Merry Christmas
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or Happy Holidays if that floats your boat...
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Dave
Phoenix, AZ